Triboelectrostatic Beneficiation de teren umplut şi Ponded cenuși zburătoare

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Triboelectrostatic Beneficiation of

Land Filled and Ponded Fly Ash

By Lewis Baker, Andrei crina, Florin Gasiorschi, and Frank Hrach

American cărbune cenuşă Asociaţiei (ACAA) anchetă anuală de producție și utilizarea cărbunelui cenuși zburătoare rapoarte că între 1966 şi 2011, peste 2.3 billion short tons of fly ash have been produced by coal-fired utility boilers.1 Of this amount approximately 625 milioane de tone au fost utilizate efectiv, cea mai mare parte pentru producţia de ciment si beton. Cu toate acestea, rămase 1.7+ miliarde tone în primul rând sunt găsite în depozitele de deşeuri sau umplute ponded impoundments. În timp ce ratele de utilizare pentru cenușa zburătoare proaspăt generată au crescut considerabil în ultimii ani, cu rate curente aproape 45%, aproximativ 40 million tons of fly ash continue to be disposed of annually. În timp ce ratele de utilizare în Europa au fost mult mai mari decât în SUA, de asemenea, au fost depozitate volume considerabile de cenușă zburătoare în depozitele de deșeuri și în sechestrele din unele țări europene.

Recent, interest in recovering this disposed material has increased, partially due to the demand for high-quality fly ash for concrete and cement production during a period of reduced production as coal-fired power generation has decreased in Europe and North America. Concerns about the long-term environmental impact of such landfills are also prompting utilities to find beneficial use applications for this stored ash.

LAND FILLED ASH QUALITY AND REQUIRED BENEFICIATION

While some of this stored fly ash may be suitable for beneficial use as initially excavated, the vast majority will require some processing to meet quality standards for cement or concrete production. Since the material has been typically wetted to enable handling and compaction while avoiding airborne dust generation, drying and deagglomeration is a necessary requirement for use in concrete since concrete producers will want to continue the practice of batching fly ash as a dry, fine powder. Cu toate acestea, assuring the chemical composition of the ash meets specifications, most notably the carbon content measured as loss-on-ignition (LOI), is a greater challenge. As fly ash utilization has increased in the last 20+ ani, most “in-spec” ash has been beneficially used, and the off-quality ash disposed. Thus, LOI reduction will be a requirement for utilizing the vast majority of fly ash recoverable from utility impoundments.

LOI REDUCTION BY TRIBOELECTRIC SEPARATION

While other researchers have used combustion techniques and flotation processes for LOI reduction of recovered landfilled and ponded fly ash, Echipament de ST & Technologies (STET) has found that its unique triboelectrostatic belt separation system, long used for beneficiation of freshly generated fly ash, is also effective on recovered ash after suitable drying and deagglomeration.

STET researchers have tested the triboelectrostatic separation behavior of dried landfilled ash from several fly ash landfills in the Americas and Europe. This recovered ash separated very similarly to freshly generated ash with one surprising difference: the particle charging was reversed from that of fresh ash with the carbon charging negative in relation to the mineral.2 Other researchers of electrostatic separation of fly ash carbon have also observed this phenomena.3,4,5 The polarity of the STET triboelectrostatic separator can easily be adjusted to allow rejection of negatively charged carbon from dried landfilled fly ash sources. No special modifications to the separator design or controls are necessary to accommodate this phenomena.

PREZENTARE GENERALĂ A TEHNOLOGIEI – SEPARAREA CARBONULUI DE CENUȘĂ DE ZBOR

În separatorul de carbon STET (Figura 1), materialul este alimentat în decalajul subţire între doi electrozi plane paralele. Particulele sunt practicate triboelectrically de interparticle contact. Carbonul încărcat pozitiv și mineralul încărcat negativ (în cenușă proaspăt generată care nu a fost udată și uscată) sunt atrase de electrozi opusi. Particulele sunt apoi măturat printr-o centură în mişcare continuă şi transmisă în directii opuse. Centura se mută particule adiacent la fiecare electrod spre capetele opuse ale separatorul. Viteza mare, de asemenea, permite cererea foarte mare, până la 36 tone pe oră pe un singur separator. Spațiu liber mic, câmp de înaltă tensiune, contra fluxul de curent, vigorous particle-particle agitation and self-cleaning action of the belt on the electrodes are the critical features

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of the STET separator. Prin controlul diverse parametrii de proces, cum ar fi viteza curelei, feed punct, şi hrana pentru animale rata, procesul de STET produce mici LOI cenuși zburătoare la conţinutul de carbon mai mică 1.5 pentru a 4.5% la feed zbura cenuşă variind în LOI din 4% la peste 25%.

Fig. 1 STET Separator processing dried, landfilled fly ash

Separatorul de proiectare este relativ simplu şi compact. O maşină concepute pentru a prelucra 40 tone pe oră este de aproximativ 30 Ft. (9 m.) lung, 5 Ft. (1.5 m.) largă, şi 9 ft., m (2.75 m.) mare. Curea si role asociate sunt doar piesele mobile. Electrozii sunt staționari și compus dintr-un material durabil în mod corespunzător. The belt is made of non-conductive plastic. Consumul de energie al separatorului este de aproximativ 1 kilowaţi-oră pe tonă de material procesat cu cea mai mare parte a puterii consumate de doua motoare centura de conducere.

Procesul este complet uscat, necesită nici materiale suplimentare, altele decât cenuși zburătoare şi produce fără emisii reziduale de apa sau aer. Materialele recuperate constau din cenuși zburătoare redusă în conţinutul de carbon la niveluri adecvate pentru utilizare ca un adaos pozzolanic în beton, şi o fracţiune de carbon de înaltă utile drept combustibil. Utilizarea a două fluxuri de produs oferă o 100% soluţie pentru problemele de eliminare cenuși zburătoare.

PROASH® RECOVERED FROM LAND FILLS

Four sources of ash were obtained from landfills: sample A from a power plant located in the United Kingdom and samples B, C, and D from the United States. All these samples consisted of ash from the combustion of bituminous coal by large utility boilers. Due to the intermingling of material in the landfills, no further information is available concerning specific coal source or combustion conditions.

The samples as received by STET contained between 15% şi 27% water as is typical for landfilled material. The samples also contained varying amounts of large >1/8 inch (~3 mm) material. To prepare the samples for carbon separation, the large debris was removed by screening and the samples then dried and deagglomerated prior to carbon beneficiation. Several methods for drying/deagglomeration have been evaluated at the pilot-scale in order to optimize the overall process. STET has selected an industrially proven, feed processing system that offers simultaneous drying and deagglomeration necessary for effective electrostatic separation. A general process flow sheet is presented in Figure 2.

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Figura 2: Process Flow Diagram

The properties of the prepared samples were well within the range of fly ash obtained directly from normal utility boilers. The most relevant properties for both the separator feeds and products are summarized in Table 2 along with recovered product.

CARBON SEPARATION

Carbon reduction trials using the STET triboelectric belt separator resulted in very good recovery of low LOI products from all four landfill fly ash sources. The reverse charging of the carbon as discussed above did not degrade the separation in any way as compared to processing fresh ash.

The properties of the low LOI fly ash recovered using the STET process for both freshly collected ash from the boiler and ash recovered from the landfill is summarized in Table 1. The results show that the product quality for ProAsh® produced from landfilled material is equivalent to product produced from fresh fly ash sources.

Tabel 1: Properties of feed and recovered ProAsh®.

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PERFORMANCE IN CONCRETE

The properties of the ProAsh® generated from the reclaimed landfill material were compared to that of ProAsh® produced from fresh fly ash generated by the utility boilers from the same location. The processed reclaimed ash meets all the specifications of ASTM C618 and AASHTO M250 standards. The following table summarizes the chemistry for samples from two of the sources showing the insignificant difference between the fresh and reclaimed material.

Tabel 2: Ash Chemistry of low LOI ash.

Strength development of a 20% substitution of the low LOI fly ash in a mortar containing 600 lb cementitious/ yd3 (A se vedea tabelul 3 Sub) showed the ProAsh® product derived from landfilled ash yielded mortars with strength comparable to mortars produced using ProAsh® from fresh fly ash produced at the same location. The end product of the beneficiated reclaimed ash would support high end uses in the concrete industry consistent with the highly valuable position ProAsh® enjoys in the markets it currently serves.

Tabel 3: Compressive strength of mortar cylinders.

PROCESS ECONOMICS

The availability of low cost natural gas in the USA greatly enhances the economics of drying processes, including the drying of wetted fly ash from landfills. Tabel 4 summarizes the fuel costs for operations in the USA for 15% şi 20% moisture contents. Typical inefficiencies of drying are included in the calculated values. Costs are based on the mass of material after drying. The incremental costs for drying fly ash for STET triboelectrostatic separation processing are relatively low.

Tabel 4: Drying costs on basis of dried mass.

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Even with the addition of feed drying costs, the STET separation process offers a low cost, industrially proven, process for LOI reduction of landfilled fly ash. The STET process for reclaimed fly ash is one-third to one-half of the capital cost compared to combustion based systems. The STET process for reclaimed fly ash also has significantly lower emissions to the environment compared to combustion or flotation based systems. Since the only additional air emission source to the standard STET process installation is a natural gas-fired dryer, permitting would be relatively simple.

VALOAREA COMBUSTIBILULUI RECUPERAT AL CENUȘII ZBURĂTOARE CU EMISII RIDICATE DE CARBON

În plus față de produsul cu emisii scăzute de carbon pentru utilizarea în beton, brand numit ProAsh®, separarea STET procesa, de asemenea, Recupereaza pierdut altfel nearse de carbon sub formă de bogate în carbon cenuși zburătoare, marcă EcoTherm™. EcoTherm™ are valoare semnificativa de combustibil şi cu uşurinţă pot fi returnate centralei electrice folosind STET EcoTherm™ Sistem de returnare pentru a reduce utilizarea cărbunelui la uzină. Atunci când EcoTherm™ este ars în cazan de utilitate, energia de ardere este convertit la presiune inalta / abur de temperatură ridicată şi apoi la electricitate la aceeaşi eficienţă ca cărbune, de obicei 35%. Conversia energiei termice recuperate în energie electrică în echipamente ST & Tehnologie LLC EcoTherm™ Sistem de retur este două-trei ori mai mare decât cea a tehnologiei competitiv, în cazul în care energiei este recuperată, grad scăzut de căldură sub formă de apă fierbinte, care este circulat la cazan apa sistem de alimentare. EcoTherm™ este de asemenea folosit ca o sursă de alumină în cuptoarele de ciment, deplasarea bauxită mai scump, care este, de obicei, transportat distanţe lungi. Utilizand EcoTherm ridicat de carbon™ cenuşă, fie la o centrala sau un cuptor de ciment, Maximizează recuperare de energie de cărbune livrate, reduce necesitatea de a mea si combustibil suplimentare la facilităţile de transport.

STET's Talen Energie Brandon Shores, RALUK SMEPA. Morrow, CORNEL Belledune, RWEnpower Didcot, FED energie Burton vest, RWEnpower Aberthaw, and the Korea South-East Power fly ash plants all include EcoTherm™ Sisteme de returnare.

STET INSTALAŢIILOR DE PROCESARE FRASIN

STET’s separation process has been used commercial since 1995 for fly ash beneficiation and has generated over 20 million tons of high quality fly ash for concrete production. Controlled low LOI fly ProAsh®, is currently produced with STET’s technology at eleven power stations throughout the U.S., Canada, Marea Britanie, Polonia, and Republic of Korea. ProAsh® cenușă zburătoare a fost aprobată pentru utilizare de peste douăzeci de autorități rutiere de stat, precum și multe alte agenții de specificații. ProAsh® a fost, de asemenea, certificată în conformitate cu Canadian Standards Association și EN 450:2005 standardele de calitate în Europa. Instalațiile de prelucrare a cenușii care utilizează tehnologia STET sunt enumerate în tabelul 5.

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Tabel 5. Fly Ash Processing facilities using STET separation technology

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Concluziile

After suitable scalping of large material, drying, and deagglomeration, fly ash recovered from utility plant landfills can be reduced in carbon content using the commercialized STET triboelectric belt separator. The quality of the fly ash product, ProAsh® using the STET system on reclaimed landfill material is equivalent to ProAsh® produced from fresh feed fly ash. The ProAsh® product is very well suited and proven in concrete production. The recovery and beneficiation of landfilled ash will provide a continuing supply of high quality ash for concrete producers in spite of the reduced production of “fresh” ash as coal-fired utilities reduce generation. Suplimentar, power plants that need to remove ash from landfills to meet changing environmental regulations will be able to utilize the process to alter a waste product liability into a valuable raw material for concrete producers. The STET separation process with feed pre-processing equipment for drying and deagglomerating landfilled fly ash is an attractive option for ash beneficiation with significantly lower cost and lower emissions compared to other combustion and flotation based systems.

Referinţe

[1]American cărbune ash cărbune produse de ardere și statistici de utilizare: http://www.acaa- usa.org/Publications/Production-Use-Reports.

[2]ST internal report, August 1995.

[3]Li,T.X,. Schaefer, J.L., Ban, H., Neathery, J.K., and Stencel, J.M. Dry Beneficiation Processing of Combustion Fly Ash, Proceedings of the DOE Conference on Unburned Carbon on Utility Fly Ash, Poate 19 20, Pittsburgh, PA, 1998.

[4]Baltrus, J.P., Diehl, J.R., Soong, Y., Sands, W. Triboelectrostatic separation of fly ash and charge reversal, Fuel 81, (2002) pp.757-762.

[5]Cangialosi, F., Notarnicola, M., Liberti, L, Stencel, J. The role of weathering on fly ash charge distribution during triboelectrostatic beneficiation, Journal of Hazardous Materials, 164 (2009) pp.683-688.

AUTHORS

Lewis Baker is the European Technical Support Manager for ST Equipment & Tehnologie (STET) based in the United Kingdom

Abhishek Gupta is a Process Engineer based at the Separation Technologies pilot plant and lab facility, STET Technical Center, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300

Dr. Florin Gasiorschi, Ph.D. is a Senior Research Scientist for ST Equipment & Tehnologie (STET) based in the New Hampshire.

Frank Hrach is Vice President of Process Engineering based at the Separation Technologies pilot plant and lab facility, STET Technical Center, 101 Hampton Ave, Needham MA 02494 +1-781-972-2300